Many enterprises and “Baby Bell” network carriers continue to use telephony equipment that establishes circuit-switched telephone calls. Circuit-switched telephone calls use dedicated circuits for the duration of the call. Typically, enterprises lease a dedicated network connection, known as a circuit-switched trunk, from network carriers. The trunk connects independent telephony systems, such as, a Private Branch Exchange (PBX) to PBX, PBX to Central Office (CO), or CO to CO. At each traffic-originating end of the call, the telephony equipment's time-division multiplexer (TDM) combines two or more data streams by assigning each data stream a different “time-slot” (i.e., channel) within a TDM frame. Each channel has a fixed length of data. Each TDM frame includes one channel per data stream where each data stream “takes a turn” to send its data over a single communication line (i.e., trunk) to the terminating end of the call. In a similar manner, the telephony equipment transmits TDM modem data and video-conferencing data. Circuit-switched trunks use various protocols to signal across these trunks. Examples of the various protocols include 64 kbps DS0 on a T1 line, E1, T3, E3, ISDN BRI, switched 56, fractional T1, frame relay, etc.
Voice over IP (VoIP) establishes an end-to-end telephone call by transmitting digitized voice samples over the same packet-switched network that carries data. Unlike circuit-switched telephone equipment, the packet-switched network does not dedicate physical circuits to the communications. Although VoIP is the trend, many enterprises and network carriers desire a slow migration path from circuit-switched services to packet-switched services such as VoIP. For enterprises and carriers, a slow migration is necessary mostly due to the cost of the transition and the perceived quality-of-service issues with VoIP.
Circuit emulation is one method of migrating from circuit-switched services to packet-switched services. Circuit emulation originated in Asynchronous Transfer Mode (ATM) standards as carriers were upgrading their network-access equipment with ATM equipment to handle the increasing loads of traffic. The ATM standards define a method to emulate a TDM circuit across an ATM network. See, ATM Forum Technical Committee, Circuit Emulation Service Interoperability Specification AF-SAA-0032.000, (September 1995).
The ATM standards provide a method for unstructured and structured service. Unstructured service encapsulates an entire bit-stream (ex. T1) in one or more packets, without knowledge of the individual channels that may exist within the bit-stream. Structured service also encapsulates the bit-stream in one or more packets, but with knowledge of the individual channels within the bit-stream.
The IETF standards body is at the forefront of Circuit Emulation Service over PSN (CESoPSN) and TDM over IP (TDMoIP), which emulate a T1, E1, T3 or E3 circuit-switched connection over a packet-switched network (PSN) such as an Internet Protocol (IP) network, an Ethernet network, or a MPLS network. The IETF standards body has published recommendations for structured and unstructured emulation of TDM bit-streams using circuit emulation. The IETF Network Working Group published “Structure-aware TDM Circuit Emulation Service over Packet Switched Network (CESoPSN),” authored by A. Vainshtein, I. Sasson, E. Metz, T. Frost, P. Pate (May 2006), found at draft-ietf-pwe3-cesopsn-07.txt. For unstructured service, the IETF Pseudo-Wire Emulation Edge to Edge (PWE3) working group published “TDM over IP,” authored by Y. Stein, R. Shashoua, R. Insler, M. Anavi, (Dec. 5, 2006), found at draft-ietf-pwe3-tdmoip-06.txt.
The majority of packet-switched networks use the Internet Protocol (IP) at the network layer. For IP networks, a Real-time Transfer Protocol (RTP) is used due to the real-time nature of the TDM voice and data transmission. The RTP packet encapsulates the TDM voice and data. The RTP protocol is defined in RFC 3550-RTP: A Transport Protocol for Real-Time Applications, Schulzrinne, et al. (July 2003).
Unfortunately, current CES recommendations increase the cost of migration from TDM to VoIP and may be error-prone. The CESoPSN recommendations statically assign bandwidth, which wastes bandwidth. The CESoPSN recommends a “Mixed amount of TDM data per packet: All the packets belonging to a given CESoPSN PW MUST carry the same amount of TDM data. This approach simplifies compensation of a lost PW packet with a packet carrying exactly the same amount of ‘replacement’ TDM data.” CESoPSN does not provide for dynamic bandwidth-management due to the recommended fixed packet sizes.
The TDMoIP recommendation requires ATM methods for dynamic bandwidth-management, but ATM methods may not be available on all networks. As discussed in the “TDM over IP” reference, “TDMoIP transports real-time streams by first extracting bytes from the stream, and then adapting these bytes. TDMoIP offers two different adaptation algorithms, one for constant-rate real-time traffic, and one for variable-rate real-time traffic.” TDMoIP provides for dynamic bandwidth-management via AAL2 methods, which require an underlying ATM layer in the network software.
The ITU has a circuit-emulation recommendation, T Y.1413. This recommendation discusses static and dynamic bandwidth-management. The dynamic bandwidth-management uses AAL2 methods, which requires an underlying ATM layer in the network software.
A thesis titled Synchronization Over Packet Switching Networks: Theory and Applications, by Raffael Noro, Thesis Number 2178, Ecole Polytechnique Federale de Lausanne (2000) recommends an out-of-band mechanism to provide dynamic bandwidth in a CESoIP network. This mechanism works by sending information regarding which circuits are still active via a separate Real-time Transmission Control Protocol (RTCP) packet. However, this extra packet introduces overhead on a possibly-congested packet-switched network. Additionally, if the RTCP packet is lost or delayed, this mechanism is error-prone. There may be a loss of synchronization at both endpoints of the network connection with regard to the number of active circuits reported. That is, there is a higher possibility of crosstalk or loss of talk-path, since the receiving endpoint does not know where the TDM data should go without additional information regarding which circuits are still active.